Electromagnetic transducers having a central armature configuration have been known in the art since at least 1929, as shown by U.S. Pat. No. 1,738,653, issued to A. H. Inglis et al. on Dec. 10, 1929. Furthermore, electromagnetic transducers having a cup-shaped permanent magnet that is inverted with respect to the end of a pole piece at which an air gap is located, have been known in the art since at least 1950, as shown by U.S. Pat. No. 2,506,609, issued to E. E. Mott on May 9, 1950.
Still further, as seen from the disclosure of U.S. Pat. No. 1,642,777 issued to W. C. Jones on Sept. 20, 1927, it has been recognized in the art since at least 1927 that magnetic circuit efficiency is a significant consideration in the design of an electromagnetic transducer. In fact, as described in the introduction of U.S. Pat. No. 3,439,130, issued to A. J. Chase et al. on Apr. 5, 1969, magnetic circuit efficiency is the prime determinant of certain important transducer characteristics, notably its physical size and weight and the size of the air gap between the transducer armature and the adjacent pole piece. Increasing the magnetic circuit efficiency permits (1) the size and weight of the transducer to be reduced and/or (2) the size of the air gap to be increased.
A transducer of small size is desirable because it permits more freedom in the design of the structure in which the transducer is to be used. A transducer of reduced weight is important where the transducer is to be held and/or carried by the user of the transducer. Small size and weight also result in reduced material usage and thereby a reduction in the cost of the transducer. Finally, an air gap of increased size is important because it increases the stability of the transducer, and it relaxes the controls that need to be exercised during its production. Consequently, the performance of the transducer is improved and the cost of manufacturing the transducer is reduced.
Despite this recognition of the benefits resulting from higher magnetic circuit efficiency, no one prior to me recognized that the combination of a central armature configuration and an inverted cup-shaped permanent magnet provides an electromagnetic transducer that has enhanced magnetic circuit efficiency.